4522b57e07
This commit introduces the ability for a client to communicate to the server features that it can support and for these features to be used in influencing the decisions that the server makes when communicating with the client. To this end we carry the features from the client to the underlying stream as we carry the version of the client today. This enables us to enhance the logic where we make protocol decisions on the basis of the version on the stream to also make protocol decisions on the basis of the features on the stream. With such functionality, the client can communicate to the server if it is a transport client, or if it has, for example, X-Pack installed. This enables us to support rolling upgrades from the OSS distribution to the default distribution without breaking client connectivity as we can now elect to serialize customs in the cluster state depending on whether or not the client reports to us using the feature capabilities that it can under these customs. This means that we would avoid sending a client pieces of the cluster state that it can not understand. However, we want to take care and always send the full cluster state during node-to-node communication as otherwise we would end up with different understanding of what is in the cluster state across nodes depending on which features they reported to have. This is why when deciding whether or not to write out a custom we always send the custom if the client is not a transport client and otherwise do not send the custom if the client is transport client that does not report to have the feature required by the custom. Co-authored-by: Yannick Welsch <yannick@welsch.lu> |
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